This invention relates to a method for fabricating capacitors in a dynamic RAM.
The capacitors in the dynamic RAM function as memory elements in each memory cell, and the quality of the dielectric film constituting the capacitors has crucial significance in improving the operational reliability of the memory elements.
In the capacitors of typical dynamic RAMs, above all, in the stack type capacitors for high integration memories, the dielectric film has a three-layer structure composed of, for example, a silicon oxide film, a silicon nitride film and a second silicon oxide film.
For increasing the capacitance while simultaneously aiming at high integration of the memory cells, it is necessary to form a material with a high dielectric constant into a thin film. A technique for such thin film formation is disclosed in, for example, "Effects of Thermal Nitridation on Underlying layer in SiO.sub.2 /SiN.sub.x Structure" in Extended Abstracts of the Japan Society of Applied Physics and Related Societies, 38th Spring Meeting, 1991, page 595, Lecture Number 28p-V-4. According to this technique, an impurity-containing polysilicon layer constituting a lower electrode of the capacitor is processed with rapid thermal nitridation to form a thermal nitride film on its surface.
Meanwhile, if a native oxide film is formed on the surface of a lower electrode layer which is one of the electrodes of the capacitor, the film quality at a boundary to the nitride layer is deteriorated to increase leakage currents and consequently the operational reliability of the dielectric layer. As a technique for forming a high reliability dielectric film, there is known a technique described in JP Patent KOKAI Publication H-3-125473 (1991), according to which a nitride film is first deposited by the low pressure CVD method on the surface of an impurity-free lower electrode layer and impurities are introduced into the lower electrode layer by ion implantation via the nitride film to prevent the native oxide film from being produced.
However, by these prior-art methods, the dielectric film cannot be improved satisfactorily in operational reliability.
First, with the method of generating the thermal nitride film on the surface of the lower electrode layer by the RTN method, a native oxide film is grown as a bottom oxide layer on the surface of the impurity-containing lower electrode layer to deteriorate the long-term reliability of the dielectric film. On this thermal nitride film, a nitride film is stacked by the low-pressure CVD method. Although the oxidation by air entrained into the CVD furnace may be prevented at this time from occurring, a native oxide film is grown during the water-washing step following a hydrofluoric acid treatment as a pre-CVD process, so that the dielectric film is deteriorated inevitably in operational reliability.
On the other hand, with the technique described in the JP Patent Publication No.H-3-125473, damages produced during 1on implantation are left in the nitride film during the time the impurities are introduced by ion implantation into the lower electrode layer which functions as a memory node, thereby again lowering the reliability of the dielectric film.